The 85mm f/1.4 ZE has a remarkable light transmission capacity. But transmitting light is not everything, and this lens has the same problem on both APS-C and full-frame sensors: it is very soft at wider apertures—but only at widest ones, since stopping down a little bit changes everything.
This is rather well summed up by its DxOMark score, showing the best low-light performance of this lens at f/2.8. This will be the key aperture. The rest is solid: good transmission, no distortion, little vignetting, and a reasonable amount of chromatic aberration contribute to making this 85mm a good candidate for portrait shooting and even for sports on an APS-C, where its focal becomes 136mm.

On a full-frame sensor (EOS 1Ds MkIII):

Everything changes at f/2.8. Opened wider than this, the lens produces a very soft image. Hardly 20lp/mm at f/1.4… this is quite poor. Stopping down to f/2 makes the center better, but still soft. Things seem dull… but when f/2.8 is reached, everything changes. The center is suddenly very sharp, around 60lp/mm, the first one-third is solid, and the edges are almost good. Between f/4 and f/5.6, the whole first one-third is very good, and the rest of the field map stands above 50lp/mm. By the way, this area is the best one for this lens. Stopped down to f/8 and above softens the center, but maintains the field around 50lp/mm.

On an APS-C sensor (EOS 7D)

On an APS-C sensor, the scenario is the same. Wider than f/2.8, the lens is not sharp enough. At f/2.8 it suddenly becomes sharp all over the field map, and remains so until stopped down to f/8. Precise and regular. Vignetting is noticeable, although very light when the lens is fully opened, but vanishes when stopped down to f/2.

This 85mm achieves its best DxOMark low-light score at f/2.8 on both APS-C and full-frame sensors, and its best mid-light score at f/4. Sharp, it produces almost no distortion, limited vignetting, and limited chromatic aberration on the edges of the field. This is a good companion for portraits, and even for sports, thanks to the crop factor that an APS-C sensor provides.

On a full-frame sensor (D3X):

As with the 50mm we studied earlier, the maximum aperture lacks definition on the whole field and the lens has to be stopped down by 2 stops to offer a central zone above 50lp/mm. At f/4, the center becomes very good, and at f/5.6, the sides of the field rise above 50lp/mm as well.

The resolution scenario is very similar here. Very soft when wide open, the lens has to be stopped down to f/4 to reach both its best resolution and to achieve good homogeneity on the whole field. Already very limited at full aperture, vignetting completely vanishes 1 stop later. At f/2, there is no visible vignetting.

Further readings for the Latest results for Zeiss lenses

To provide photographers with a broader perspective about mobiles, lenses and cameras, here are links to articles, reviews, and analyses of photographic equipment produced by DxOMark, renown websites, magazines or blogs.

Carl Zeiss is one of those almost mythical lens makers. Focusing on quality, its lenses almost always rely on complex optical formulas. Their price reflects this involvement in quality, and logically Zeiss lenses are on the same shelves as the very high-end Canon or Nikon models. We have tested 14 of them so far, ranging from the ultra-wide-angle Distagon T18mm f/3.5 to the much heavier Makro Planar T100mm f/2.